Manage parallel port output

Hi, Sanforall. As a straightforward digital logic problem, you will need more than 8 bits to get 256 output signals, unless you just want to have only one on at a time.

Usually, with decoders and data latches, you'd have the data output, then you'd also need control bits. You don't have quite enough to get to 256, because you need both data and control.

If you need mass quantities of I/O bits from a printer port, and you don't need microsecond resolution, you might want to look at daisy-chaining 32 ea. 74HC595 ICs. These are 8-bit serial input - parallel output shift registers with data latches. You can do this with printer port pins to spare. If you have 32 of these ICs (not totally unrealistic with decent layout, and they're around $1 USD apiece), you can shift your 32 8-bit data words out the parallel port one bit at a time, using the clock signal to shift forward after each bit, then using the latch clock to assert the new data at the output once your shifting is complete.

If you're building this, keep your wiring runs short to avoid having noise munging your data. You should have this assembly as close to the printer port as possible. No ten foot printer cables. Also, use good layout practices on the board, and use a decoupling capacitor for each IC. Power glitches will cause big headaches here.

As a practical matter, programming is very easy and straightforward to do with DOS and in assembler. But if you're using Windows, the software end of it might get to be a bit of a problem. One good resource for utilizing the PC printer port is Jan Axelson's "Printer Port Complete", available from many hobbyist sources such as Jameco, or from the Lakeview Research webste

Look at the webpage for more hints. The book also has a software CD which will help with programming in VB or other languages.

Good luck Chris

To control 256 output signals from an 8-bit data port, you need

32 x 8-bit latches and a 1-of-32 selector. That selector takes 5 data bits. The printer control port can supply those, but may take some finagling since some pins are inverted and some need pull-ups, etc. So instead, you can use one bit of the control port to select a "mode": In one state, the data port output will go to an address latch, whose output goes to the multiplexer address pins. In the other state, the data port output will go to the multiplexer input, and hence be directed to the proper output latch. You can use a second control bit to do the actual strobing of address or data, once the mode is selected. This way, you can (in principle) have up to 256 x8-bit latched outputs... 65536 in all! And you can update any 8-bit subset of these in only a few operations, copmpared to a complete serial approach.

Best regards,

Bob Masta dqatechATdaqartaDOTcom D A Q A R T A Data AcQuisition And Real-Time Analysis

Home of DaqGen, the FREEWARE signal generator

Sanforall, I have just started on a similar project, and I am having a hard time getting my c code to send anything to the port. Have you been successful with your code? If so, would you please post it so that I could use it.

thanks, flank

"sanforall" schreef in bericht news: snipped-for-privacy@y43g2000cwc.googlegroups.com...

Ah, well I advise to learn something more about the parallel port. A well known source is

Pay special attention to the Enhanced Parallel Port as IMHO this is the most easy way to implement what you need. On almost all modern PCs the parallel printer port can be setup for EPP in the BIOS. Keep in mind that you also may need a device driver. Information on the same site.

As for the hardware you use the address strobe to clock the required address in a LS364 like buffer. Its outputs has to drive an address decoder wich can be build using LS138 like decoders. You need to decode 32 of the 256 possible addresses. AND each decoded address line with the data strobe of parallel port to clock data in 32 LS364 type data buffers.

petrus bitbyter

I've got some pages below that have some simple parallel port control setups. If your OS is win NT/2K/XP, you will need to have a driver that allows user level access to the hardware (I use userport). Getting 256 seperate outputs from 8 bits might be a bit tricky. If you can also use the parallel port control pins in addition to the data pins, you can control a lot more than 256 outputs. I like to use the 74hct259 chips which are cheap. Bottom link shows a different chip setup that controls ~388 strings of christmas lights. You can write simple control applications using notepd and batch files.

what do you want from these 256 signals - do you want to be able to have more than one of them on at a time etc...

Bye. Jasen

[...]

with the right glue logic the strobe and ack pins on the printer port could be used to clock the data into the chips. making writing to the device as simple as writing binary data to a real printer. no assembler needed.

Bye. Jasen

Yes. I kind of read this to be a homework problem. Even with a SPP, this is very doable (although a little unwieldly). To get 32 CS you'd need an extra 4 ICs (e.g. HC138), plus some gates.

Cheers Chris